The development of the measuring techniques using laser beams has been promoted recently. Examples of such techniques include a technique for detecting the amount of a specific substance in a test sample by using the absorption intensity of a laser beam (laser spectroscopic measurement), and a technique for using a laser beam to detect minimal variations in a subject to be measured (interferometer). These measuring techniques are desired to have reduced laser emission line widths.
A laser beam is also used as pump light for generating an electromagnetic wave at terahertz frequencies. Such a laser beam, too, is desired to have a reduced emission line width.
Patent Documents 1 and 2 each describe a laser system with a ring resonator in which a rare-earth doped fiber is used as a gain medium. Especially Patent Document 1 describes that the rare-earth doped fiber configuring the ring resonator is provided with a fiber Bragg grating in order to reduce the line width of output light.
Patent Document 1: Japanese Translation of PCT Application No. 2000-501244
Patent Document 2: Japanese Translation of PCT Application No. 2008-511182
A novel method needs to be implemented to reduce the line width of output light at low cost.